1. Field of the Invention
The present invention relates to a method for fabricating a semiconductor memory device and, more particularly, to a method for fabricating a capacitor of a semiconductor memory device.
2. Description of the Related Art
A high capacitance capacitor is more advantageous for the purpose of securing reliability of a semiconductor device. Especially for memory devices among the semiconductor elements, a capacitor is required whose capacitance is as high as possible so as to preserve refresh due to automatic extinction of a data value. Accordingly, every effort has been made to increase capacitance in the memory products.
Capacitance may be increased by enlarging a surface area of a storage electrode or by using a dielectric film whose dielectric constant is great. However, most dielectric films are still developing except for an NO film and an ONO film. Accordingly, capacitance is generally increased by enlarging a surface area of the storage electrode.
A surface area of a storage electrode may be enlarged with a stack type, a cylinder type, a fin type, or a trench type. The trench type and the stack type are mainly used. For the trench type, a capacitor is made by depositing a storage electrode layer after digging a trench on a semiconductor substrate. The trench type may enlarge the surface area of a capacitor, but have a complex and difficult isolation or processing procedure.
On the contrary, the stack type has a simple processing procedure. Furthermore, it is not complex to increase the height of the stack for the purpose of increasing capacitance. However, this increased height makes lithography difficult during a subsequent metal wiring process. That is, a step between a cell and a peripheral region becomes so great that a focus margin of lithography is reduced, thereby making the metal wiring thin or generating a cut-off or bridge. In view of solving the foregoing matters, it is required that a storage electrode have a small step between a cell and a peripheral region. Since a dielectric film having a very high dielectric constant is not generally available yet, an HSG (hemispherical grain) method is generally used to enlarge a surface area of a capacitor.
FIG. 1A to FIG. 1C illustrate sequential processes of a method for fabricating a capacitor of a prior semiconductor memory device.
Referring to FIG. 1A, a device isolation layer 11 for defining an active region and an inactive region is formed on a semiconductor substrate 10. A gate electrode layer 103 is formed on a gate oxide layer over the semiconductor substrate 10. An oxide layer 14, which is formed from a lower oxide layer 14a and an upper oxide layer 14b and serves as an insulating layer, is formed on the semiconductor substrate 10, including the gate electrode layer 13, where a transistor is formed.
A bit line 15 is formed in the oxide layer 14, between the lower oxide layer 14a and the upper oxide layer 14b. The oxide layer 14 is etched to form a contact hole 16 by using a contact hole formation mask until a surface of the semiconductor substrate 10 is exposed. The contact hole 16 is filled with a conductive material such as a polysilicon, thereby forming a plug 17 which is electrically connected to the semiconductor substrate 10.
Referring to FIG. 1B, a polysilicon layer 18 serving as a conductive layer is formed on the oxide layer 14 including the plug 17. The polysilicon layer 18 is etched by using the storage electrode formation mask, thereby forming a storage electrode 18 which is electrically connected to the plug 17. An HSG layer 19 enlarging a surface area of a capacitor is formed on a surface of the storage electrode 18.
Referring to FIG. 1C, a capacitor dielectric film 20 is formed on the oxide layer 14 including the HSG layer 19. A polysilicon layer 21, which is doped to serve as an upper capacitor electrode, is deposited on the capacitor dielectric film 20 to form a capacitor.
As mentioned above, a stack type capacitor has merits in that the processing procedure is simple and a throughput is good. However, it is difficult for the stack type capacitor to have enough capacitance value by scaling-down the device design rule according to the high density of a device and form a pattern whose size may prevent a bridge with an adjacent capacitor during formation of an HSG layer.
The reason is as follows. A space between patterned storage electrodes should be wide enough so as to grow an HSG layer. Accordingly, a delicate lithography process is needed and the height of an electrode should be increased so as to atone for a surface area, which may greatly influence the subsequent metal wiring process. As the HSG layer is grown on a surface of a patterned storage electrode, as shown in symbol xe2x80x98Axe2x80x99 of FIG. IB, a part of the HSG layer is connected between the storage electrodes, thereby generating a minute micro-bridge. Since the micro-bridge results in device failure, reliability may be degraded.
The present invention was made in view of the above problem. A feature of a method and a device consistent with the present invention preferably produces a capacitor of a semiconductor memory device which can prevent a micro-bridge by securing a sufficient margin between storage electrodes and enlarge a surface area of a storage electrode.
In accordance with the present invention, the method includes the steps of forming a first insulating layer on a semiconductor substrate, etching the first insulating layer to form a contact hole, filling the contact hole with a conductive material to form a plug electrically connected to the semiconductor substrate, forming a second insulating layer on the first insulating layer including the plug, etching the second insulating layer until the plug and a part of the first insulating layer are exposed to form a storage electrode opening, forming a conductive spacer on both sidewalls of the storage electrode opening, each conductive spacer being electrically connected to the plug, and forming an HSG (hemispherical grain) on the surfaces of the conductive spacers and the plug.
In a preferred embodiment of the present invention, the method further comprises a step of forming an etch stopper layer on the first insulating layer after formation of the first insulating layer.
According to the preferred embodiment, a second insulating layer is formed on a first insulating layer including a plug. The second insulating layer is etched until the plug and a part of the first insulating layer are exposed, thereby forming a storage electrode opening. A conductive spacer is formed on both sidewalls of the storage electrode opening, wherein each conductive spacer is electrically connected to the plug. An HSG layer is formed on the surfaces of the conductive spacer and the plug.
A capacitor recording to the present invention enables the HSG layer to grow on an internal wall of a storage electrode, thereby preventing a micro-bridge bet storage electrodes resulting from abnormal growth or over-growth of the HSG layer.